A common method for the manufacture of asphalt shingles is the production of a continuous strip of asphaltic shingle material followed by a shingle cutting operation which cuts the material into individual shingles. In the production of asphaltic strip material, either an organic felt or a glass fiber mat is passed through a coater containing liquid asphalt to form a tacky, coated asphaltic strip. Subsequently, the hot asphaltic strip is passed beneath one or more granule applicators which apply the protective surface granules to portions of the asphaltic strip material. Typically, the granules are dispensed from a hopper at a rate which can be controlled by making manual adjustments on the hopper. In the manufacture of colored shingles, two types of granules are employed. Headlap granules are granules of relatively low cost for portions of the shingle which are to be covered up. Colored granules or prime granules are of relatively higher cost and are applied to the portion of the shingle which will be exposed on the roof.
To provide a color pattern of pleasing appearance the colored shingles are provided in different colors, usually in the form of a background color and a series of granule deposits of different colors or different shades of the background color. These highlighted series of deposits, referred to as blend drops, are typically made from blenders which comprise a series of granule containers connected to feed rolls. The length and spacing of each mixture on the sheet is dependent on the speed of the feed roll, the relative speed of the sheet and the length of time during which the drop is made.
Not all of the granules applied to the hot, tacky, coated asphaltic strip adhere to the strip, and, typically, the strip material is turned around a slate drum to invert the strip and cause the non-adhered granules to drop off. These non-adhered granules, which are known as backfall granules, are usually collected in a backfall hopper and reused on the shingle.
One of the problems with typical granule application equipment is that the feeder rolls depend on mechanical movement (rotation) to index to the next position to enable another blend drop to fall onto the moving coated asphalt sheet. This requirement for mechanical action has inherent limitations which prevent a precise beginning and ending to the blend drop. Also, once the mechanical action takes place, there is a short time lag as gravity takes effect on the granules. Consequently, there is a limit to the sharpness of the blend drops on the shingle. As shingle manufacturing lines go up in speed the lack of sharpness is accentuated, and the distinction between the blend drop and the background color becomes fuzzy. The lack of sharpness puts a severe limitation on the kinds of designs and color contrasts which can be applied to the shingle.
A recently developed improved method for depositing granules onto the moving coated asphalt sheet uses a pneumatic control to provide a relatively high degree of preciseness in depositing the granules. The newly developed method provides relatively instantaneous control of the flow of granules. The flow of granules is started, stopped and controlled by providing pneumatic pressure changes in a buffer chamber positioned adjacent an accumulation of granules in a granule nozzle. It has been found, however, that although the pneumatically controlled granule blend drop apparatus provides a very sharp leading edge for a blend drop, it produces a fuzzy or less sharp trailing edge for the blend drop. An improved process would provide for blend drops having both a sharp leading edge and a sharp trailing edge.